There is known an internal combustion engine that generates active species in a combustion chamber, and thus, accelerates combustion of mixed gas. For example, Patent Document 1 discloses an internal combustion engine of this kind.
More specifically, the internal combustion engine disclosed in Patent Document 1 causes a spark discharge to be generated in a discharge gap of the spark plug and microwaves to be radiated toward the discharge gap. A plasma generated in the discharge gap receives energy from the microwave pulse. As a result thereof, electrons in a plasma region are accelerated and a volume of the plasma increases. Many active species are generated accompanied with the generation of the plasma.